Universal Mobile Telecommunication System (UMTS) is a widely used wireless communication system nowadays. In UMTS, like in any other wireless system, frequency errors, or offsets, between the transmitter (e.g. a base station) and receiver (e.g. a user terminal) can be introduced in the transmitter and due to Doppler shift in the wireless channel. Such frequency offsets can lead to a communication performance degradation at the receiver, such as an increase in bit-error rate and a decrease in throughput. It is thus desired that these frequency offsets be compensated, e.g., reduced to the minimum, at the receiver in order to achieve best performance.
Developers and operators of wireless communication systems are under the constant pressure of providing an ever-increasing data rate, although radio spectrum is a limited resource. A solution to this need is the High-Speed Downlink Packet Access (HSDPA) protocol, which allows legacy UMTS networks to offer higher data transfer speeds and capacity. HSDPA in legacy UMTS systems is based on single cell, or single carrier, operation. The frequency offset between a transmitter and a receiver is thus compensated at the receiver for single carrier operation.
To achieve even higher data rates, the 3GPP standard body has approved the Dual Cell (or Dual Carrier) High Speed Downlink Packet Access (DC-HSDPA) protocol, which can improve the bit rate in terms of peak rate and average bit rate from the perspective of the user terminal. In DC-HSDPA, data targeted for a particular receiver is modulated onto two distinct HSDPA carriers at different frequencies and then transmitted together to the receiver.
Dual-Carrier HSDPA is included in Release 8 of 3GPP; other variations such as Dual Band HSDPA and DC-HSUPA are included in Release. 9. HSDPA-specific issues for dual-carrier operation are addressed in the 3GPP document “Technical Specification Group Radio Access Network; Dual-Cell HSDPA Operations”, 3GPP, TR 25.825 V. 1.0.0.
Utilization of more than two carriers has also been proposed. Multi-carrier (MC) operation involves jointly scheduling two or more HSDPA carriers to increase the peak data rate per user terminal and increase the utilization of available frequency resources by multiplexing carriers in the CELL DCH state.
In DC- or MC-HSDPA there are two types of carriers: The first carrier type known as the “Anchor carrier” carries all the legacy physical channels including DPCH/F-DPCH, E-HICH, E-RGCH, E-AGCH, PICH, SCCPCH, AlCH etc. The other carrier type relates to “supplementary” carriers, which carry a reduced set of physical channels in order to reduce signaling overhead.
According to 3GPP Technical Specification “Universal Mobile Telecommunications System (UMTS); Multiplexing and channel coding (FDD)” 3GPP TS25.212 version 8.5.0 Release 8, the required base-station frequency accuracy for single carrier in a UMTS system is 0.05 parts per million (ppm) for macro base stations. Considering two individual carriers in the case of DC-HSDPA, the relative frequency error at the base station reference points could be up to 0.1 ppm. Currently there is no 3GPP requirement on base station relative frequency error but it can be assumed that a base station which uses the same source for frequency generation can have a relative frequency error lower than 0.1 ppm since the error introduced in the source can be excluded from the final value of the user terminal-relative frequency error.
Frequency offset estimation and compensation at a receiver is normally performed by a so-called Automatic Frequency Control (AFC) function. Because user terminals generally rely on inexpensive oscillators to provide a time and/or frequency reference, they must repeatedly estimate the time and frequency offset between a received signal and an on-board reference signal to correctly and efficiently receive and decode received data. Obviously, this will also be true for multi-carrier extensions of today's wireless communication systems.